A resin composition comprising a fluorine-containing resin containing a vinylidene fluoride unit and a polyfunctional monomer is known to undergo electron radiation crosslinking to provide heat-shrinkable tubing as described, e.g., in JP-B-43-29665 (the term "JP-B" as used herein means an "examined published Japanese patent application").
It is also known to provide a hot-melt adhesive layer on the inner wall of heat-shrinkable tubing, which is melted by heating simultaneously with heat shrinkage of the tubing to thereby fill the gap between an adherend surface and the tubing and, at the same time, hold the tubing in place. Many products utilizing such a two-layer structure have been placed in the market.
Examples of hot-melt adhesives generally used as the inner layer of conventional two-layer heat-shrinkable tubing include polyolefin resins, polyamide resins, polyester resins, butyl rubbers, etc. However, these materials are poor in chemical resistance and heat resistance. Further, the inner layer comprising such an adhesive acts to reduce flame retardance of the tubing as a whole. Addition of a flame retardant to the hot-melt adhesive to restore flame retardance leads to reductions in physical properties as an adhesive.
The conventional adhesives have poor chemical resistance and undergo degradation or denaturation in an acidic or alkaline atmosphere, unavoidably resulting in a reduction of sealing effect. Furthermore, the conventional adhesives have poor heat resistance and easily suffer foaming or loss of softness and eventually fall off upon being heated in temperatures higher than necessary or when used for a long time even at temperatures below the melting point.